Electronic circuits that provide a measure of distance from the circuit to an object are known in the art, and may be exemplified by system 10 FIG. 1. In the generalized system of FIG. 1, imaging circuitry within system 10 is used to approximate the distance (e.g., Z1, Z2, Z3) to an object 20, the top portion of which is shown more distant from system 10 than is the bottom portion. Typically system 10 will include a light source 30 whose light output is focused by a lens 40 and directed toward the object to be imaged, here object 20. Other prior art systems do not provide an active light source 30 and instead rely upon and indeed require ambient light reflected by the object of interest. FIG. 1 may be said to exemplify what is described in assignee Canesta, Inc.'s U.S. Pat. No. 6,323,942 to Bamji entitled “CMOS-Compatible Three-Dimensional Image Sensor IC” (2001).
Various fractions of the light from source 30 may be reflected by surface portions of object 20, and is focused by a lens 50. This return light falls upon various detector devices 60, e.g., photodiodes or the like, in an array on an integrated circuit (IC) 70. Devices 60 produce a rendering of the luminosity of an object (e.g., 10) in the scene from which distance data is to be inferred. In some applications devices 60 might be charge coupled devices (CCDs) or even arrays of CMOS devices.
CCDs typically are configured in a so-called bucket-brigade whereby light-detected charge by a first CCD is serial-coupled to an adjacent CCD, whose output in turn is coupled to a third CCD, and so on. This bucket-brigade configuration generally precludes fabricating processing circuitry on the same IC containing the CCD array. Further, CCDs provide a serial readout as opposed to a random readout. For example, if a CCD range finder system were used in a digital zoom lens application, even though most of the relevant data would be provided by a few of the CCDs in the array, it would nonetheless be necessary to readout the entire array to gain access to the relevant data, a time consuming process. In still and some motion photography applications, CCD-based systems might still find utility.
As noted, the upper portion of object 20 is intentionally shown more distant that the lower portion, which is to say distance Z3>Z2>Z1. In a range finder autofocus camera environment, one might try to have devices 60 approximate average distance from the camera (e.g., from Z=0) to object 10 by examining relative luminosity data obtained from the object. In some applications, e.g., range finding binoculars, the field of view is sufficiently small such that all objects in focus will be at substantially the same distance. But in general, luminosity-based systems do not work well. For example, in FIG. 1, the upper portion of object 20 is shown darker than the lower portion, and presumably is more distant than the lower portion. But in the real world, the more distant portion of an object could instead be shinier or brighter (e.g., reflect more optical energy) than a closer but darker portion of an object. In a complicated scene, it can be very difficult to approximate the focal distance to an object or subject standing against a background using change in luminosity to distinguish the subject from the background. In such various applications, circuits 80, 90, 100 within system 10 in FIG. 1 would assist in this signal processing. As noted, if IC 70 includes CCDs 60, other processing circuitry such as 80, 90, 100 are formed off-chip.
Unfortunately, reflected luminosity data does not provide a truly accurate rendering of distance because the reflectivity of the object is unknown. Thus, a distant object surface with a shiny surface may reflect as much light (perhaps more) than a closer object surface with a dull finish.
Other focusing systems are known in the art. Infrared (IR) autofocus systems for use in cameras or binoculars produce a single distance value that is an average or a minimum distance to all targets within the field of view. Other camera autofocus systems often require mechanical focusing of the lens onto the subject to determine distance. At best these prior art focus systems can focus a lens onto a single object in a field of view, but cannot simultaneously measure distance for all objects in the field of view.
In general, a reproduction or approximation of original luminosity values in a scene permits the human visual system to understand what objects were present in the scene and to estimate their relative locations stereoscopically. For non-stereoscopic images such as those rendered on an ordinary television screen, the human brain assesses apparent size, distance and shape of objects using past experience. Specialized computer programs can approximate object distance under special conditions.
Stereoscopic images allow a human observer to more accurately judge the distance of an object. However it is challenging for a computer program to judge object distance from a stereoscopic image. Errors are often present, and the required signal processing requires specialized hardware and computation. Stereoscopic images are at best an indirect way to produce a three-dimensional image suitable for direct computer use.
Many applications require directly obtaining a three-dimensional rendering of a scene. But in practice it is difficult to accurately extract distance and velocity data along a viewing axis from luminosity measurements. Nonetheless many applications require accurate distance and velocity tracking, for example an assembly line welding robot that must determine the precise distance and speed of the object to be welded. The necessary distance measurements may be erroneous due to varying lighting conditions and other shortcomings noted above. Such applications would benefit from a system that could directly capture three-dimensional imagery.
Although specialized three dimensional imaging systems exist in the nuclear magnetic resonance and scanning laser tomography fields, such systems require substantial equipment expenditures. Further, these systems are obtrusive, and are dedicated to specific tasks, e.g., imaging internal body organs.
Various techniques for acquiring and processing three dimensional imaging have been developed by assignee herein Canesta, Inc. of San. Jose, Calif. For example, in addition to the '942 patent to Bamji, U.S. Pat. No. 6,522,395 (2003) to Bamji et al. discloses Noise Reduction Techniques Suitable for Three-Dimensional Information Acquirable with CMOS-Compatible Image Sensor ICs; and U.S. Pat. No. 6,512,838 to Rafii et al. (2003) discloses Methods for Enabling Performance and Data Acquired from Three-Dimensional Image Systems.
Applications are known in the art that seek to acquire three dimensional images using mechanical devices. For example, scanning laser range finding systems raster scan an image by using mirrors to deflect a laser beam in the x-axis and perhaps the y-axis plane. The angle of deflection of each mirror is used to determine the coordinate of an image pixel being sampled. Such systems require precision detection of the angle of each mirror to determine which pixel is currently being sampled. Understandably having to provide precision moving mechanical parts add bulk, complexity, and cost to such range finding system. Further, because these systems sample each pixel sequentially, the number of complete image frames that can be sampled per unit time is limited. It is understood that the term “pixel” can refer to an output result produced from one or more detectors in an array of detectors.
In summation, there is a need for improved detection devices and methods for use with a system that can produce direct three-dimensional imaging, preferably using circuitry that can be fabricated on a single IC using CMOS fabrication techniques, and requiring few discrete components and no moving components. Optionally, the system should be able to output data from the detectors in a non-sequential or random fashion. Very preferably, such system should require relatively low peak light emitting power such that inexpensive light emitters may be employed, yet the system should provide good sensitivity.
The present invention provides such a system.